1. Field of the Invention
This invention relates to a lock-out logic circuit for use in conjunction with feedback inverter systems, and in particular, but not exclusively, resonant feedback inverter systems.
2. Description of the Prior Art
Various inverter systems have previously been proposed to convert the output of a direct current (d.c.) power source to an alternating current (a.c.). Generally, these systems employ a pair of commutating load rectifiers which are either self-commutated of the resonant-feedback type, or of the auxiliary impulse commutated type which are commutated by means of additional commutating rectifiers and an impulse forming circuit. Since this invention is directed to a lock-out circuit for a resonant feedback inverter system, the following discussion is limited to a description of this type of system. Examples of auxiliary impulse commutated inverter systems are found in U.S. Pat. Nos. 3,641,421; 3,852,657; and 3,919,620.
An example of a self-commutated resonant feedback inverter circuit was published by Mapham, "An SCR Inverter with Good Regulation and Sine Wave Output," IEEE Transaction on Industry and General Applications, IGA-3, No. 2, March/April, 1967, and is shown in FIG. 1 of this application. The operation of this circuit is as follows. When SCR1 is triggered, current flows from the supply E charging up capacitor C to a voltage approaching 2E. The current then reverses and flows back to the supply E via diode D1, and C discharges. During the reverse current flow, the rectifier SCR1 becomes reversed biased and is thereby turned off. Then SCR2 is triggered on and a similar cycle occurs in the lower half of the circuit, with a negative voltage pulse occuring across the capacitor C. SCR1 is then again triggered after SCR2 becomes reversed biased, and the cycle continues.
In reality, the output of the inverter circuit shown in FIG. 1, i.e. the voltage across capacitor C, is transformer coupled to a load either for isolation or voltage transformation purposes. The load across the output capacitor C therefore includes L2, the primary inductance of the load transformer, and R and C2 which represent the reflected load. However, the reflected load capacitance C2 may lower the resonant frequency of the inverter circuit sufficiently to cause an inverter malfunction if switching of the load or a change in the load occurs at a certain time in the half cycle. Specifically, if either of the load rectifiers, SCR1 or SCR2, is delayed in turning off while the other load rectifier commences conduction, simultaneous conduction by both load rectifiers occurs, which effectively short circuits the load and is known as "rectifier shoot-through." A shoot-through results in current levels far greater than those existing during normal operation, and can result in destruction of the load rectifiers.